The present invention relates to rotary fluid pressure devices used as hydraulic motors, and more particularly, to such motors in which the fluid displacement mechanism is a gerotor gear set, and the motor valving is of the spool valve type.
Rotary fluid pressure devices which include a gerotor gear set as the fluid displacement mechanism are typically used as low-speed, high-torque motors. Such gerotor motors have traditionally been classified as being either of the "spool valve" type, or of the "disk valve" type. In a spool valve gerotor motor, the valving is accomplished at a cylindrical interface between a spool valve and a spool bore defined by the surrounding housing. In a disk valve type, the valving is accomplished at a flat, transverse planar interface of a disk valve and stationary valve member.
In certain applications for low speed, high torque gerotor motors, one of the performance criteria which is especially important to the vehicle manufacturer is the mechanical efficiency at start-up of the motor, under load. This is also sometimes referred to as the starting torque efficiency of the motor, or simply the "starting efficiency". As is well known to those skilled in the art, efficiency of a hydraulic motor is expressed as a percentage, and mathematically, is the mechanical horsepower output of the motor divided by the hydraulic horsepower input to the motor.
As is also well known to those skilled in the art, the starting efficiency of a spool valve gerotor motor is typically better than that of a disk valve gerotor motor, primarily because of the amount of torque required to begin rotating the disk valve, which is biased into engagement with its stationary valve member. This would suggest that spool valve gerotor motors would be preferred for such applications where starting efficiency is an important factor.
However, as is also known to those skilled in the art, the typical prior art spool valve motor involves a pair of annular grooves, generally defined on the outer surface of the spool valve, with one of the grooves being connected to the inlet (high pressure) port and the other groove being connected to the outlet (low pressure) port. Extending axially from the grooves is a plurality of axial valve passages (also referred to as "timing slots") with the high pressure and low pressure axial passages being arranged in an interdigitated pattern. These axial passages then engage in commutating communication with passages in the valve housing which communicate with the volume chambers of the gerotor gear set, also in a manner well known to those skilled in the art.
Unfortunately, the prior art, interdigitated arrangement of the valve passages results in an extremely long, generally serpentine-shaped interface between high pressure and low pressure, and because there must, by definition, be a clearance between the valve spool and the spool bore, there is ample opportunity for cross port leakage which reduces volumetric efficiency.
In many of the applications of the type noted above where starting efficiency is important, there is also a need for two-speed capability, i.e., the ability to operate in a low-speed, high-torque mode on the work site, as well as the ability to operate in a high-speed, low torque mode during transport between work sites. Although U.S. Pat. No. 3,778,198, incorporated herein by reference, illustrates a spool valve motor having two-speed capability, it is believed that prior to the present invention, there has not been a two-speed spool valve gerotor motor in commercial production.